Speedo Calibration 2006 Ranger

[absolutesantaja]

I've done a bit of digging on here but i haven't found anyone else with a problem like mine. I have a 2006 Ranger 2.3l that i just purchased. The truck has about 33,000 miles on it. For some reason the speedometer is about 3-4 mph slow. I've taken it to two different dealers and they say there is nothing wrong with it. I first checked it with my gps but today i finally picked up my dad's scantool and hooked up to the trucks computer so i could see what speed the computer say's the truck is doing and the computer shows the correct speed but its still slower than what the dash reads. Is there some way to adjust the gauge. It seems to be off the same amount no matter what speed I'm going.

Thanks
Shawn